The present invention relates to a method of controlling the recovery of metal from a solution in an electrolytic cell by plating (or deposition), onto an electrode thereof. The invention finds particular, though not exclusive application in the recovery of silver from photographic processing solutions.
For convenience the invention will be discussed, by way of example only, with reference to photographic solutions used in black and white processing.
Photographic material, in sheet or roll film form, is processed in several stages. These stages include chemical development, fixing of the image, washing and drying.
The role of the photographic solution with fixing ability is to form soluble salts of any unexposed silver halide grains in the emulsion of the sensitised material. As more material is processed, the fixing solution becomes seasoned with soluble silver ion complexes. These complexes can reduce the fixing ability of the solution and can affect the quality of the final image. Ultimately the solution can become too loaded with silver and it would then be necessary to replace it with fresh solution. However environmental legislation is increasingly putting stricter limitations on the disposal of waste material bearing silver. Consequently, attention is increasingly being paid to safe and efficient recovery of the silver either by recovery of silver from the effluent, which is then disposed of, or by in-line treatment in which the silver bearing solutions are withdrawn from a processing tank, passed through an electrolytic cell and returned to the processing tank. The advantages of in-line electrolytic recovery of silver include
i) the lifetime of the fixing solution can be extended,
ii) the rate of fixing of the image can be increased,
iii) the rate of replenishment of the solution with fresh chemicals can be reduced,
iv) treatment of the process effluent from the processing is facilitated, and
v) the value of the silver recovered is economically worthwhile.
As with any electrochemical process, however, poor control can do more harm than good. Silver recovery is no exception. When a silver recovery cell is operated efficiently the only cathodic reaction to occur is the reduction of silver ions to silver metal. This is governed by the potentials at this electrode. If too high a potential is applied, side reactions can occur which in turn lead to the production of unwanted by-products, for example silver sulphide can be formed as a fine precipitate in the solution (sulphiding). The recovery of silver is often therefore a compromise between the need for high plating currents, and consequently at higher potentials, to maintain low silver concentrations in the processing tank and the requirement of safe operation. Some large scale commercial units employ a third electrode (most commonly a reference electrode) or a silver electrode, so as to improve the efficiency of operation. However these add to the cost and problems can arise with calibration of the equipment and electrical drift. It is possible, however, with a reference electrode, for example, to limit the cathode potential so as not to exceed the potential for the formation of silver sulphide under any recovery conditions. EP 0598144 employs a third, pH electrode and the potentials of the three electrodes are controlled so as to avoid sulphiding. In addition to the disadvantage of cost of such a three electrode system, the maximum rate of removal of silver is itself limited by the fact that the potential of the cathode is kept constant.
The generally cheaper two electrode control system (using just the anode and cathode) relies on a knowledge of the cell currents and voltages for the means of control. The most common method is to use a threshold level beyond which, (above which for voltage or below which for current) it is deemed no longer suitable to recover further silver. For example when silver is recovered at a constant current, the plating voltage rises as the concentration of silver falls. In this instance the voltage is reflecting both a change of conductivity in the solution and the change of the potentials at the cathode and anode. A disadvantage of this control method is its lack of robustness as the threshold level chosen for switch off is not necessarily a suitable or even safe place to switch off for all operating conditions. This problem is exacerbated by the fact that each processor to which silver recovery is attached has a specific combination of operating parameters causing variability in the concentration of the constituents of the solution. Such operating parameters are, for example:
i) film exposure, and thus the proportion of silver that is removed by the fixer,
ii) film type, and thus the quantity of silver available for development and fixing,
iii) film throughput, i.e. how much film is processed per hour,
iv) processor type, and thus the amount of solution that is carried into the fixing stage from the development stage and the amount of oxidation that takes place,
v) the chemical composition of replenisher solution used in the various stages of the processing, and
vi) the rate at which the processing solutions are replenished.
The voltage necessary to supply a certain current through a fixer solution at a given silver concentration will show a strong dependence on the pH of the solution, the concentration of sulphite and/or thiosulphate in the solution, the solution temperature and the rate at which it flows through the cell. Therefore, the specific operating parameters of the processor have a significant effect on plating conditions in the electrolytic cell through their effect on the fixer solution.
The film throughput is an important factor of the operating parameters as it governs the recovery current that must be supplied to maintain a low silver concentration in the fixer tank. Thus, although many small, commercial low-cost silver recovery units achieve crude control of plating at low currents with reasonable efficiency, these units would be unsuited to the higher recovery currents needed to maintain adequately low silver concentrations in higher throughput operations. A key geometrical design parameter, which governs maximum recovery current, is the cathode area. A large cathode facilitates high currents. However, to minimise the footprint of the silver recovery unit, a small cathode area is desirable. A requirement of an improved control system, therefore, is the ability to control the operation of the cell at relatively high current densities safely.
EP 0856597 discloses a process for monitoring the electrolyte circulation in an electrolysis cell where the electrolysis is performed by a constant current or electrode potential. The method disclosed takes multiple measurements of an electrolysis parameter over a period of time and uses these measurements to evaluate whether an error is occurring.
U.S. Pat. No. 6,187,167 discloses methods for the efficient control of a silver recovery unit containing an anode and cathode as the only electrodes. The methods are preferably operated at a constant current whilst analysing the changing plating voltages which arise from the varying silver concentrations as silver is either removed from the solution through plating or added to the solution through the processing of film. These two control methods enable the recovery process to adapt to changes in the solution by adjusting the plating current to the maximum level at which desilvering is efficient. However a disadvantage of both these methods lies in the relative nature of their operation. The control methods require two measurements to be made at different silver concentrations so that a comparison can be made. If presented with a solution of unknown concentration it is not possible for these control systems to find the most suitable de-silvering current with which to begin the de-silvering process if the silver concentration is remaining constant. A test current must be applied to cause a silver concentration change. This difficulty can be overcome if recent historical data has been stored for the same system under very similar operating conditions. There are however occasions where this approach will not be possible, for example, when recovery is performed in batch mode or when the system is used for the very first time in an in-line configuration.
Batch mode de-silvering, refers to the de-silvering of a solution under isolated conditions, i.e. the fixer solution is de-silvered once and not reused. De-silvering as referred to above, has referred to the in-situ or in-line desilvering of the solution in a processor fixer tank. In such cases the fixer solution is continuously re-circulated between the processor tank and the recovery cell during de-silvering.
In cases where the silver concentration is unknown and there is no previous historical data, it is prudent to use small test currents in case the silver concentration is low. If the silver concentration is high, on the other hand, such small currents are found to make the control methods quite insensitive to changes in silver concentration. In the high silver case for an on-line system, it is possible that the silver concentration may start to increase as film is processed at a rate faster than the small test current is desilvering the solution. In this situation, the silver in the fixer can rise to very high concentrations where fixing performance may be affected and where the silver concentration of the wash effluent can exceed discharge limits.
It is an aim of the invention to provide an electrolytic recovery process in which an electrolytic recovery unit operating at high current density can be controlled with a two electrode control system such that there is no sulphiding, under any operating parameters. The process should be able to assess the most suitable current to apply to an unknown solution when starting the recovery process.
A method is required that can quickly give an absolute indication of the most suitable current that can be applied to an unknown solution. This is especially the case when the silver recovery unit is being used in the following situations:
i) for the first time
ii) after an interruption to the process
iii) if the solution is being de-silvered in batch mode.
According to the present invention there is provided a method of controlling the recovery of metal from solution flowing through an electrolytic cell containing a cathode and an anode by deposition onto the cathode thereof as current flows through the cell between the cathode and the anode under the action of a voltage thereacross, comprising the steps of
a) applying one of a first constant current or voltage at a first average solution flow rate,
b) changing the solution flow rate to a second average flow rate for a first period of time,
c) monitoring the other of the current or voltage during the period of time, and
d) obtaining information from the monitored current or voltage, the information being used to control the rate of recovery of metal from the solution.
The invention further provides a method comprising the steps of applying one of a first constant current or voltage at a first average solution flow rate,
changing the solution flow rate to a second average flow rate for a first period of time,
monitoring and storing the other of the current or voltage during the first time period,
restoring the solution flow rate to the first flow rate,
changing the one of the current or voltage to a second constant current or voltage,
changing the solution flow rate to the second average flow rate for a second period of time,
monitoring and storing the other of the current or voltage during the second period of time,
obtaining information from the stored currents or voltages, and
selecting and applying a current or voltage from the first and second current or voltage levels in response to the information obtained thereby to control the rate of recovery of metal from the solution.
Preferably the second average flow rate is substantially zero.
The method according to the invention is both less costly and more convenient than conventional processes known in the art. Cost is reduced by not using ancillary reference or silver sensing electrodes. This also improves convenience by eliminating the problems of electrode drift and fouling which may require the recalibration or replacement of the ancillary electrodes.
The method of the invention can give rise to control systems or methods that can be used to give efficient, adaptive silver recovery for all solutions at any known or unknown concentration. These control systems would therefore be suitable for de-silvering fixer solutions either in-line or in batch mode. Furthermore these control methods are relatively cheap and simple to implement and allow high currents to be reached very quickly, when appropriate. The methods may also be used in addition to the basic xcex94V operating system, as disclosed in U.S. Pat. No. 6,187,167, in cases where there is an unknown solution which could have high silver concentration, a circumstance where the xcex94V method becomes less sensitive or insensitive. Alternatively, the methods could replace the xcex94V method altogether and form a control system in their own right. The xcex94V method is not well suited to batch mode operation where it is always the case that the starting solution contains an unknown, and probably high, level of silver.
The invention improves the robustness of the xcex94V control system allowing it to deal with solutions with high silver concentration where sensitivity is lost under normal (high flow) operating conditions. A further benefit of this is that the time taken for the control system to determine that a high plating current may be safely used is greatly reduced. This results in less possibility of the silver concentration rising to unacceptably high levels in the fixer tank.